Several methods are available for the frequency measurement:
1) pulse counting within the prescribed time, described in papers [1, 2, 3, 4]: PA0 2) period measurement based on the elapsed time between pulses, described in papers [5, 6, 7, 8, 9]: PA0 3) measurement of time duration of variable number of pulses, described in U.S. patents [10, 11, 12]: PA0 4) combined pulse counting and time measurement, using the start and stop of measurement synchronized with measured frequency, described in [13, 14, 15]: PA0 5) constant elapsed time (CET) measurement of time and pulse counting, using a capture register operation synchronized with measured frequency, described in papers [16, 17]: PA0 6) double buffered (DB) time measurement and pulse counting using constant sampling time, described in papers [18, 19, 20, 21]:
[1] T. J. Maloney and F. L. Alvarado, "A digital method for DC motor speed control," IEEE Trans. Ind. Electron. Contr. Instrum., IECI-23, No. 1, pp. 44-46, February 1976. PA1 [2] S. C. Lin and S. J. Tsai, "A microprocessor-based incremental servo system with variable structure," IEEE Trans. Ind. Electr., IE-31, No. 4, pp. 313-316, November 1984. PA1 [3] K. Ohishi, M. Nakao, K. Ohnishi, and K. Miyachi, "Microprocessor-controlled DC motor for load-insensitive position servo system," IEEE Trans. Ind. Electr., IE-34, No. 1, pp. 44-49, February 1987. PA1 [4] K. Kuboki and M. Ohtsu, "An Allan variance real-time processing system for frequency stability measurements of semiconductor lasers," IEEE Trans. Instrum. Measur., IM-39, No. 4, pp. 637-641, August 1990. PA1 [5] B. Szabados, N. K. Sinha, and C. D. diCenzo, "High-resolution precision digital tachometer," IEEE Trans. Instrum. Meas., IM-21, No. 2, pp. 144-148, May 1972. PA1 [6] C. D. diCenzo, B. Szabados, and N. K. Sinha, "Digital measurement of angular velocity for instrumentation and control," IEEE Trans. Ind. Electron. Contr. Instrum., IECI-23, No. 1, pp. 83-86, February 1976. PA1 [7] E. E. Wallingford and J. D. Wilson, "High-resolution shaft speed measurements using a microcomputer," IEEE Trans. Instrum. Meas., IM-26, No. 2, pp. 113-116, June 1977. PA1 [8] B. Nabibullah, H. Singh, K. L. Soo, and L. C. Ong, "A new digital speed transducer," IEEE Trans. Ind. Electron. Contr. Instrum., IECI-25, No. 4, pp. 339-342, November 1978. PA1 [9] E. P. McCarthy, "A digital instantaneous frequency meter," IEEE Trans. Instrum. Meas., Vol. IM-28, No. 3, pp. 224-226, September 1979. PA1 [10] J. Valis, "Means for Frequency/Digital Conversion," U.S. Pat. No. 3,928,798. PA1 [11] J. Takahashi, T. Shibata, S. Naito, and K. Tokuyama, "Skid control method," Hitachi, Ltd., U.S. Pat. No. 4,398,260. PA1 [12] G. F. Pierce, "Frequency determining apparatus," Canadian General Electric Company, Canadian Patent 1,144,986; U.S. Pat. No. 4,420,809. PA1 [13] T. Ohmae, T. Matsuda, K. Kamiyama, and M. Tachikawa, "A microprocessor controlled high-accuracy wide-range speed regulator for motor drives," IEEE Trans. Ind. Electr., IE-29, No. 3, pp. 207-211, August 1982. PA1 [14] T. Matsuda, M. Kanno, K. Saito, and T. Sukegawa, "A Microprocessor-based motor speed regulator using fast response state observer for reduction of torsional vibration," IEEE Trans. Ind. Appl., vol. IA-23, No. 5, pp. 863-781, September 1987. PA1 [15] J.-P. Fauvet, J. Parisel, "Speed determining process and a device for implementing same," La Telemecanique Electrique, U.S. Pat. No. 4,683,545. PA1 [16] R. Bonert, "Digital tachometer with fast dynamic response implemented by a microprocessor,"IEEE Trans. Ind. Appl., vol. IA-19, No. 6, pp, 1052-1056, December 1983. PA1 [17] R. Bonert, "Design of a high performance digital tachometer with a microcontroller," IEEE Trans. Instrum. Meas., vol. IM-38, No. 6, pp. 1104-1108, December 1989. PA1 [18] M. Prokin, "Double buffered wide-range frequency measurement method for digital tachometers," IEEE Trans. Instrum. Meas., vol. IM-40, No. 3, pp. 606-610, June 1991. PA1 [19] M. Prokin, "Speed measurement using the improved DMA transfer method," IEEE Trans. Ind. Electr., vol. IE-38, No. 6, December 1991. PA1 [20] M. Prokin, "Dynamic response of a frequency measuring system," IEEE Trans. Instrum. Meas., vol. IM-41, No. 3, June 1992. (accepted) PA1 [21] M. Prokin, "Dynamic response of the double-buffered frequency measurement method," IMTC/92 Conference Record, Secaucus, N.J., May 12-14, 1992. (accepted)
The error in 1) increases for low frequencies. The error in 2) increases for high frequencies. The method 3) requires prior information pertaining to the actual frequency range. The methods 4), 5) and 6) provide less error than the methods 1), 2) and 3). The sampling period in the methods 2), 3), 4) and 5) is variable and depends on measured frequency. The variation in sampling time in 4) is less than 1:3 through the frequency range, while the duration of an input pulse period is less than the time interval determined by the interval timer. The similar variation is within 1:2 for the method 5). The repetitive start, stop and reinitialization of counters also reduces the sample rate in 4). The repetitive enabling and disabling of the capture register function limits the maximum measured frequency and the frequency range in 5), which is also limited by the number of bits inside the timer.
On the contrary, the DB method 6) provides a high accuracy and a fast dynamic response over an extremely wide range of frequencies using constant sampling period. The detailed comparison between the DB method 6) and other methods will be performed after the description of the embodiment of the DB method, as an object of this invention.